[sdiy] Finding the correct IC...

Neil Johnson nej22 at hermes.cam.ac.uk
Wed Jun 19 22:57:07 CEST 2002

> There is a circuit called Latch (D-type, S/R, ...) that according to a
> friend can help me with this problem, as it triggers on a pulse and
> stays High/low until it is Reset (Set/Reset). ... I want ONE pin to do
> BOTH... does anyone know of a circuit that can do this for me?

Ok, so firstly you'll need to debounce the switch otherwise every (pos or
neg) edge will toggle your sequencer stage and drive you nuts trying to
program it.  I'd initially suggest a simple RC filter and Schmitt buffer.

Now, to do the actual toggling, if you have an asynchronous design then a
common J-K flip-flop, with the J and K inputs wired to logic '1' and the
debounced button connected to the CLK input, the output being Q (if you
didn't have the debounce there then each positive edge from the switch
would toggle the FF several times before settling...a right pain...)

If you're doing a synchronous design (unlikely, but...) then you'll need a
D-type to sample the button signal, a D-type to store the previous sample,
and a T-type and a couple of gates to implement the edge-detector and
toggle function.  Unless you're using an FPGA best to stick with asynch
for now.


Neil Johnson :: Computer Laboratory :: University of Cambridge ::
http://www.njohnson.co.uk          http://www.cl.cam.ac.uk/~nej22
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